The invention relates to the manufacture of rotary drill bits for use in drilling or coring deep holes in subsurface formations. In particular, the invention is applicable to rotary drill bits of the kind comprising a bit body having a shank for connection to a drill string and an inner channel for supplying drilling fluid to the face of the bit, and where the bit body carries a plurality of so-called preform cutting elements, each element having a hard cutting face formed of polycrystalline diamond or other superhard material.
In one commonly used method of making rotary drill bits of the above-mentioned type, the bit body is formed by a powder metallurgy process. In this process a hollow mould is first formed, for example from graphite, in the configuration of the bit body or a part thereof. The mould is packed with powdered matrix-forming material, such as tungsten carbide, which is then infiltrated with a metal alloy, such as a copper alloy, in a furnace so as to form a hard matrix.
Polycrystalline diamond preforms are available which are thermally stable up to the infiltration temperature, typically about 1100.degree. C. Such thermally stable diamond materials are supplied, for example, by the General Electric Company under the trade name "GEOSET", and by DeBeers under the trade name "SYNDAX 3". Such materials may be mounted in the surface of the bit body, in the infiltration process, by locating the thermally stable performs in pockets formed in the interior surface of the mould, and then packing the matrix-forming material around the preforms.
One common form of thermally stable preform comprises a thick tablet of triangular shape and of constant cross-section. Another is of cubic shape. A desirable arrangement for such preforms at the bit body surface is for the preform to be so mounted in the surface that one apex of the triangle, or one corner of the cube, projects from the surface of the drill bit, the general plane of the triangle, or face of the cube, extending generally radially with respect to the central axis of rotation of the bit. Behind the preform is an upstanding tooth comprising a portion of the matrix body of the bit which is of the same general V-section as the projecting portion of the preform and which extends rearwardly thereof to provide back support for the preform. In order to form such a support during moulding of the bit body, the pocket in the mould in which each preform is located is elongate and of generally V-section. The preform is then located adjacent one end of the pocket so that the rest of the pocket fills with matrix-forming material to provide the upstanding tooth on the finished bit body.
The pockets in the mould may conveniently be formed by automatic machining operations under computer control. However, problems arise in conveniently forming, in such computer controlled machining operations, pockets of the required shape to receive thermally stable preforms of the kind referred to above. Since the elongnate pocket is of V-section it is most conveniently machined by use of a rotating conical forming tool which extends perpendicularly to the interior surface of the mould and is traversed along the required longitudinal axis of the pocket. However, such a method results in an elongate V-sectioned pocket which is concavely curved, in semi-conical manner, at its two ends. Accordingly, since the triangular preform has flat faces, when the preform is located in the pocket adjacent one end, there is a semi-conical gap between the front face of the preform and the adjacent curved end of the pocket. If this gap is allowed to fill with matrix-forming material during the process of moulding the bit body, this will result, in the finished drill bit, in their being an upstanding pad of matrix in front of the preform, as well as the required elongate matrix back support on the rearward side of the preform. Such an arrangement is shown, for example, in U.S. Pat. No. 4,529,047.
This may be undesirable in that it may interfere with the effective cutting operation of the preform. It has been proposed to overcome this problem by filling the gap in the pocket in the mould with some removable filler before the mould is packed with matrixforming powder, but this involves extra labour, and hence cost in preparing the mould for the infiltration process.
It is therefore desirable to be able to machine in the mould, using a computer controlled tool, an elongate pocket of general V-section which has at one end a flat end face against which the front face of the preform may abut when the preform is located in the pocket, so that matrix-forming material then only fills the pocket on the rearward side of the preform. The present invention provides a method of forming such pockets.